1. Field of the Invention
The present invention relates to an image processing device and a method for controlling the same, and in particular, to an image processing device and a method for controlling the same which employ a technique which is applied to filter processing in an image processing section of an image forming device, such as a color image copier or the like, which handles image signals of different sampling rates.
2. Description of the Related Art
In a conventional color image copier, three-channel image signals such as RGB signals are read out by a charge coupled device (CCD) or the like at the same sampling rate, and are processed in an image processing section at a latter stage.
Thus, at a filter included in the image processing section, frequency characteristics, such as a cut-off frequency and the like, which are appropriate for only that sampling rate are set.
Moreover, when a line sensor (CCD), which outputs image signals of plural channels having different sampling rates, is used as image input means, a Nyquist frequency changes in accordance with the sampling rate. Thus, functions such as elimination of moire and the like cannot be realized with the same filter factor.
In particular, when an image, to which a screen (dots) has been applied, is inputted such as a document manuscript by dot printing, at a time of a low sampling rate, the Nyquist frequency is lower than the screen frequency. Sampling is carried out with a periodic configuration of the manuscript image being bent back with the Nyquist frequency in the center.
On the other hand, at a time of a high sampling rate, there are cases in which the Nyquist frequency is higher than the frequency of the periodic configuration of the input image (the number of dot lines of the printed manuscript).
In this way, when the Nyquist frequency changes in accordance with the sampling rate of the input image signal, at the filter processing section, functions such as the elimination of moire and the like cannot be realized when filter processing is carried out at the same filter factor. Thus, a separate, excessive un-sharpness processing is required, and the problem arises that this is not always preferable from the standpoint of cost performance.